1. Field of the Invention
The present invention relates to a lead overlay sensor with an improved current path and, more particularly, to a read head wherein the shunting of a portion of a sense current through hard bias layers adjacent end portions of the sensor to lead overlay regions of the sensor is substantially prevented so that substantially all of the sense current is conducted to the lead overlay regions by first and second leads.
2. Background Information
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has write and read heads, a suspension arm and an actuator arm. When the disk is not rotating the actuator arm locates the suspension arm so that the slider is parked on a ramp. When the disk rotates and the slider is positioned by the actuator and suspension arms above the disk, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the actuator arm swings the suspension arm to place the write and read heads over selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The write and read heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a spin valve sensor for sensing the magnetic field signals from the rotating magnetic disk. The sensor includes a nonmagnetic electrically conductive first spacer layer sandwiched between a ferromagnetic pinned layer structure and a ferromagnetic free layer structure. An antiferromagnetic pinning layer typically interfaces the pinned layer structure for pinning a magnetic moment of the pinned layer structure 90E to the air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the magnetic disk. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. A magnetic moment of the free layer structure is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or bias point position in response to positive and negative magnetic field signals from the rotating magnetic disk. The quiescent position, which is preferably parallel to the ABS, is the position of the magnetic moment of the free layer structure with the sense current conducted through the sensor in the absence of field signals.
The thickness of the spacer layer is chosen so that shunting of the sense current and a magnetic coupling between the free and pinned layer structures are minimized. This thickness is typically less than the mean free path of electrons conducted through the sensor. With this arrangement, a portion of the conduction electrons are scattered at the interfaces of the spacer layer with the pinned and free layer structures. When the magnetic moments of the pinned and free layer structures are parallel with respect to one another scattering is minimal and when their magnetic moments are antiparallel scattering is maximized. Changes in scattering changes the resistance of the spin valve sensor as a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layer structures. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in the resistance of the sensor as the magnetic moment of the free layer structure rotates from a position parallel with respect to the magnetic moment of the pinned layer structure to an antiparallel position with respect thereto and R is the resistance of the sensor when the magnetic moments are parallel.
In addition to the spin valve sensor the read head includes nonconductive nonmagnetic first and second read gap layers and ferromagnetic first and second shield layers. The spin valve sensor is located between the first and second read gap layers and the first and second read gap layers are located between the first and second shield layers. In the construction of the read head the first shield layer is formed first followed by formation of the first read gap layer, the spin valve sensor, the second read gap layer and the second shield layer. Spin valve sensors are classified as a bottom spin valve sensor or a top spin valve sensor depending upon whether the pinned layer is located near the bottom of the sensor close to the first read gap layer or near the top of the sensor close to the second read gap layer. Spin valve sensors are further classified as simple pinned or antiparallel (AP) pinned depending upon whether the pinned layer structure is one or more ferromagnetic layers with a unidirectional magnetic moment or a pair of ferromagnetic AP layers that are separated by a coupling layer with magnetic moments of the ferromagnetic AP layers being antiparallel to one another. The AP pinned layers may be pinned in their magnetic orientation by an antiferromagnetic (AFM) layer or may be self-pinned. Spin valve sensors are still further classified as single or dual wherein a single spin valve sensor employs only one pinned layer and a dual spin valve sensor employs two pinned layers with the free layer structure located therebetween.
It is important that the free layer be longitudinally biased parallel to the ABS and parallel to the major planes of the thin film layers of the sensor in order to magnetically stabilize the free layer. This is typically accomplished by first and second hard bias magnetic layers which abut first and second side surfaces of the spin valve sensor. Unfortunately, end portions of the free layer abutting the hard bias layers are over-biased and become very stiff in their response to field signals from the rotating magnetic disk. The stiffened end portions can take up a large portion of the total width of a sub-micron sensor and can significantly reduce the amplitude of the sensor. It should also be understood that a narrow track width is important for promoting the track width density of the read head. The more narrow the track width the greater the number of tracks that can be read per linear inch along a radius of the rotating magnetic disk. This enables an increase in the magnetic storage capacity of the disk drive.
There is a need to reduce the stiffening of the magnetic moment of the free layer when longitudinally biased. This has been accomplished by employing a lead overlay (LOL) scheme wherein first and second leads overlay top surfaces of the first and second hard bias layers thence overlay and are electrically connected to first and second top surface end portions of the sensor, which portions are referred to in the art as first and second lead overlay (LOL) regions. The purpose of this scheme is to render the first and second LOL regions substantially insensitive to field signals from the rotating magnetic disk so that only a central portion of the sensor between the first and second LOL regions is sensitive to such field signals. Unfortunately, because of electrical contact between the first and second lead layers and the first and second hard bias layers, a portion of the sense current is shunted through the first and second hard bias layers to the LOL regions of the sensor. In a typical read head the sheet resistance of the hard bias layers is approximately twenty times the resistance of the lead layers. This means that most of the sense current should be carried by the first and second lead layers to the sensor. However, most sensors have a high resistance cap layer which is located between the first and second leads and the LOL regions of the sensor. A typical cap layer is tantalum (Ta). The cap layer forces a portion of the sense current to go through the first and second hard bias layers to the LOL regions of the sensor. This causes the LOL regions to become partially sensitive to the field signals, which sensitivity causes side reading of tracks adjacent the track that is being read by the central portion of the sensor. Accordingly, the magnetic read width (MRW) of the read head is larger than that desired. The MRW is typically greater than the physical track width (TW) wherein the physical track width is the distance between the first and second lead layers which is also the distance between the first and second LOL regions. There is a strong-felt need to reduce the MRW which, in turn, reduces the aforementioned side reading.